The present invention relates generally to measuring apparatuses, and more particularly to a measuring apparatus that uses as the light source a light emitting source in an extreme ultraviolet (“EUV”) region or an X-ray region.
The recent developments of X-ray, soft X-ray, and EUV optics have expanded their applications in a variety of field. In particular, a field of evaluation of optical elements and the like for the EUV light has attracted attention. The polarization of light needs to be considered in evaluating the reflectance of an optical element through irradiation, because the property that an optical element indicates generally differs from the polarization of light. As shown in FIG. 18, polarized light with an electric field vector perpendicular to the paper, and against a specular surface 1000a is defined as s-polarized light, and polarized light with an electric field vector perpendicular to the s-polarized light and vertical to a wave number vector is defined as p-polarized light. Where Is is intensity of the s-polarized light and Ip is intensity of the p-polarized light, the degree of polarization P is given in the following equation:P=(Is−Ip)/(Is+Ip)  (1)Here, FIG. 18 is a schematic view showing how the light is polarized.
For example, there is a multilayer mirror having high reflectance in the X-ray or soft X-ray regions, but the reflectance of such a multilayer reflecting mirror differs with polarization of incident light. FIG. 19 shows reflectance characteristics obtained by calculation about a multilayer mirror as a five layer pairs pair with a thickness of 9.6 nm combining the molybdenum (Mo) and silicon (Si) layers when the incident angle of light is set to 42.6°. This figure adopts the horizontal axis as wavelengths of light incident on the multilayer mirror, and the longitudinal axis as reflectance of the multilayer mirror. In other words, as to light that mixes p-polarized light and s-polarized light in it, the values differ depending on the degree of polarization, and therefore it is difficult to measure reflectance with accuracy. Thus, to accurately measure reflectance, it is necessary to separate linearly polarized light as the p-polarized light or as the s-polarized light.
Conventionally, however, it has not been easy to create linearly polarized light from a light source of non-polarized light for use in measurement, and thus the linearly polarized light with a high degree of polarization and planes of polarization that can be switched.
For example, as an apparatus that accurately measures reflectance, a reflectometer has been used conventionally that uses a synchrotron light source. FIG. 20 is a schematic view of a reflectometer 2000 that uses a synchrotron light source. The reflectometer 2000 uses synchrotron radiation from a bending magnet of a light source 2100. Synchrotron radiation from the bending magnet has linearly polarized light with an electric field vector in a plane of an electron orbit. Therefore, via a subsequent optical system 2200, light separated as the p-polarized light or the s-polarized light is irradiated onto a measured object 2300, and thus it is possible to measure reflectance using a detector 2400 in consideration of polarized light. However, since the reflectometer 2000 using a synchrotron light source has synchrotron radiation from the light source 2100 that is always linearly polarized into the plane of the electron orbit, there is no other choice than changing the direction of the measured object 2300 so as to shift the direction of polarization onto the measured object 2300 (i.e., choose between s-polarized light and p-polarized light). Accordingly, in order to change the direction of the measured object 2300, it has been necessary to turn the chamber containing the measured object 2300, which usually weighs several hundred kilograms, thus making it very difficult to choose between s-polarized light and p-polarized light. Furthermore, a synchrotron light source itself has been of a very large scale, and highly expensive.
Thus, a reflectometer using a laser producing plasma light source (hereinafter called an LPP light source), which is small-sized and low-priced compared to the synchrotron light source, is proposed in the Proceedings SPIE Vol. 4144 (2000), pp. 76–81, “Development of an EUV Reflectometer using a laser-plasma X-ray source.” Such an apparatus determines multilayer parameters by curve-fitting data obtained in measurement. As the polarization of light from a light source is assumed to be random, and s-polarized light is estimated to be 5% from the calculation of an optical system arrangement, there still remains uncertainty in the degree of polarization, and errors are produced in reflectance, thus being unable to determine highly precise multilayer film parameters.
Further, a reflectometer, which circumvents the above problems by inserting a transmission-type multilayer polarizer having excellent polarization characteristics, has been proposed in the Proceedings of SPIE Vol. 1720 (1992) pp. 190–194, “Soft-x-ray polarization measurement with a laboratory reflectometer.” Such an apparatus, though it employs an LPP light source, makes it possible to measure reflectance, separating polarized light by attaching a transmission-type multilayer polarizer with molybdenum (Mo) and silicon (Si) laminated. However, considering transmittance, it is necessary to contain the thickness of the multilayer polarizer (the thickness including a multilayer film and a plate) within several hundred nm or less, and accordingly, it is difficult to manufacture and easy to break, and has heat-resisting problems. In addition, when a non-polarized soft X-ray with a wavelength of 12.8 nm transmits a polarizer with molybdenum (Mo) and silicon (Si) laminated in 41 layers, a transmittance intensity ratio of s-polarized light and p-polarized light is only as much as 0.2, thus being unable to obtain a high degree of polarization.
In order to obtain a desired polarization state, use of a reflection polarizer that is easy to manufacture and excellent in durability is proposed in Rev. Sci. Instrum. 66 (2) pp. 1598–1600, February 1995, “Performance evaluation of a soft x-ray quadruple reflection circular polarizer.” However, this merely turns linearly polarized light into circularly polarized light using a structure of four multilayer mirrors, and thus, it cannot be used for polarization dependent characterization of a highly precise sample.
Use of two polarizers structured in three multilayer mirrors that rotate around the optical axis is proposed in the Rev. Sci. Instrum. 66 (2) pp. 1923–1925, February 1995, “Polarization characterization of circularly polarized vacuum-ultraviolet and soft-x-ray helical undulator radiation.” However, this is an apparatus that measures characteristics of a synchrotron light source and degrees of circularly polarized light, thus being used for different purposes.
For the above apparatuses, use of a large-scale structure is unavoidable to perform highly accurate measurement in consideration of polarization and by using synchrotron. A structure suitable for highly accurate measurement by using an LPP light source has not been realized that is a small-sized and low-priced light source using non-linearly polarized light.